1. Field of the Invention
This invention relates to a method of reconstructing a high quality image in a digital processing system for reconstructing an image which can be comprehended by people from data taken by a spaceborne or airborne synthetic aperture radar (hereinafter called "SAR").
2. Description of the Prior Art
An SAR that provides an image of high spatial resolution in a microwave band through clouds has been drawing an increasing attention as a sensor for photographing the land surface in the field of remote sensing using a satellite or an airplane.
The outline of data processing of the data collected by an SAR will be first described. For detail, refer to Ando, "Synthetic Aperture Radar and Indirect Measuring Technique", Measurement and Control, Vol. 22, No. 2, for example.
In the reception images of an SAR, one point of the original image is distributed with a point spread function h(x, y), and can not be utilized as such. Here, x represents the range and y represents the azimuth. The data that is spread inside the reception images is first compressed in the range direction and then, in the azimuth direction. Range compression is effected by correlation with point pattern data for each line of the image data. If correlation is executed as such, however, an extremely prolonged processing time is necessary, so that a fast Fourier transformer (hereinafter called "FFT"), a complex multiplier, and an inverse fast Fourier transformer (hereinafter called "IFFT") are used to improve the processing time.
On the other hand, the following three standpoints have been taken in a conventional SAR image reconstruction system because the point spread function of the data in the azimuth direction on the reception image might change with the position of the point image in the azimuth direction.
The first assumes that the point spread function inside the scene to be processed is constant, and then executes the processing analogous to the range compression described above. However, this system is not free from the problem that if the change of the point image is great, the resulting output image is in focus only partially inside the scene. [Refer to J. R. Bennette, I. G. Cumming, "Digital SAR Image Formation Airborne and Sattelite Results, Proc. 13th Int. Symp. Remote Sensing of Environment, pp. 337-360 (1979)].
The second divides the scene to be processed into fine segments, and determines the point spread function for each divided image. Focusing can be obtained if the rate of division is increased, but a necessary fixed calculation quantity irrelevant to the processing size exerts adverse influences so that the calculation time becomes greater as a whole.
Unlike the range compression described above, the third system executes calculation of correlation in a real space while changing the point spread function. Though this system provides the best focal accuracy, the calculation time becomes imcomparably longer than the system using FFT. [Refer to RAE (Royal Aircraft Establishment): ESPF Study and System Implementation Manual].
Thus, the conventional SAR image reconstruction systems only leave the choice of whether the calculation speed is improved by use of FFT while neglecting the change of the point image in the azimuth direction, or precise focusing and high image quality are obtained without using FFT at the sacrifice of the calculation speed.